Hydropeaking impacts on the Lez river and studies to define mitigation measures

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OATAO is an open access repository that collects the work of Toulouse researchers and

makes it freely available over the web where possible.

This is an author-deposited version published in :

http://oatao.univ-toulouse.fr/

Eprints ID : 11291

To cite this version : Baran, Philippe and Boucard, Florian and

Boyenval, Elise and Cassan, Ludovic and Garmendia, Laurent and

Hurel, Gregory and Prel, Pauline Hydropeaking impacts on the Lez

river and studies to define mitigation measures. (2012) In:

Eawag/Wasser-Agenda 21 Workshop on Hydropeaking, 19 June 2012 -

19 June 2012 (Zurich, Switzerland).

Any correspondance concerning this service should be sent to the repository

administrator:

staff-oatao@listes-diff.inp-toulouse.fr

1

Hydropeaking impacts on the Lez river and studies to

define mitigation measures

Presented by Courret D1

(dominique.courret@imft.fr) In alphabetical order : Baran P1_{, Boucard F}3 _{Boyenval E}3_{, Cassan}

L1_{, Garmendia L}2_{, Hurel G}1_{& Prel P}1

3 2

1 The Lez River downstream Eylie Power plant

- Mountain stream in Pyrenees - Slope : ≈ 0.3% - Width : 3-5 m - Brown trout population 2

Lez river

reservoir at high altitude (≈1900 m,

diverted basin area : ≈ 12 km²)

Intermediate basin with flow

reconstitution (≈ 16 km²)

Hydropeaks restitution at an altitude of 870 m, on an nearly natural hydrological regime

- Basin area : ≈ 28 km² - Mean discharge : ≈ 1 m3_/s

- Width : 3-5 m

- Maximum turbine discharge : 3.6 m3_/s

This causes high flows compared to

stream size Affected reach

3

Lez river

• Hydropeaks visualisation, at Bordes station (around 15 km downstream hydropeaks restitution [212 km²], mean discharge 7.1 m3_/s)

• Most hydropeaks’ amplitudes corresponds to maximum turbine flow

• Base flow estimates downstream Eylie power plant range down to 0.3-0.5 m3_{/s (30-50% of mean discharge)}

Hydropeaks characterization

Lez at Bordes station Year 2004 0 4 8 12 16 20 24 28 32 1/1 31/1 1/3 31/3 30/4 30/5 29/6 29/7 28/8 27/9 27/10 26/11 26/12 Date F lo w ( m 3/s )

Lez at Bordes station Year 2004 0 1 2 3 4 5 6 7 8 1/8 5/8 9/8 13/8 17/8 21/8 25/8 29/8 Date F lo w ( m 3/s ) 4

Lez river

Number of hydropeaks per trout life stages

0 50 100 150 200 250 300 350 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 Fry growth Larval Emergence Intragravel Spawning

5

• Low trout densities and biomass compared to non-affected streams

(183 and 312 ind./100 m, in Isard and Riberot tributaries with comparable size])

Biological issues linked to hydropeaks

0+ 1+ 2+ 0 20 40 60 80 100 120 140 160 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 D e n s it y o f tr o u t (i n d ./ 1 0 0 m ) 0 500 1000 1500 2000 2500 3000 3500 B io m a s s o f tr o u t (g / 1 0 0 m ) Density Biomass 6

Lez river

Fry recruitment negatively related to the number of hydropeaks during emergence period

Biological issues linked to hydropeaks

0 20 40 60 80 100 120 0 10 20 30 40 50 60

Number of hydropeaks during emergence period

D e n s it y o f tr o u t fr y ( in d ./ 1 0 0 m )

7

Lez river

• Low influence of fry (0+) recruitment level on juvenile (1+) densities the next year

Biological issues linked to hydropeaks 0 5 10 15 20 25 30 35 40 45 50 55 0 5 10 15 20 25 30 35 40 45 50 55

Density of trout 0+ year n (ind./100 m)

D e n s it y o f tr o u t 1 + y e a r n + 1 ( in d ./ 1 0 0 m ) 0 5 10 15 20 25 30 35 40 45 50 55 0 5 10 15 20 25 30 35 40 45 50 55

Density of trout 1+ year n (ind./100 m)

D e n s it y o f tr o u t 2 + y e a r n + 1 ( in d ./ 1 0 0 m )

It’s suspected that habitat conditions during hydropeaks are limiting for juveniles and adults

• No influence of juvenile densities on 2+ trout densities the next year

8

Lez river

• Relicensing process includes blocks placement to mitigate hydropeaks impacts

• 2D hydraulic modelling of 4 stations to test several modalities for blocks placement

assessment of efficiency thank to microhabitat method

Studies to improve habitat conditions during hydropeaks

9

• Hydraulic conditions searched in block wakes :

– Minimum water depth of 20-30 cm at low flow

– Maximum water velocity of 20 cm/s in wake during hydropeaks – Provide shelter

• 2 modalities tested for blocks placement :

Studies to improve habitat conditions during hydropeaks

“Isolated” block + groynes Rows of blocks forming porous sills

10

Lez river

Studies to improve habitat conditions during hydropeaks

Flows velocities (left) and habitat values for adult (right) during an hydropeak on station 2

Presently

With placement of

blocks and groynes Presently

With placement of blocks and groynes

11

Lez river

• Hydraulic modelling results confirm that habitat conditions during hydropeaks can be limiting for trout population, more than low flow periods

• Block placement can be a solution to improve habitat conditions during hydropeaks

Studies to improve habitat conditions during hydropeaks

Evolution of habitat values for adult stage in station 2

Flow (m3/s)

Presently With block placement

Base flow During hydropeak

12

Lez river

• High flows conditions during hydropeaks (maximum flow over 4 times the mean discharge)appear to be limiting for the trout population.

• Block placement can be a solution to improve habitat conditions during hydropeaks, but this need to be dimensioned (size and position of blocks)
2D hydraulic modelling can be a useful tool.

• Solution not yet implemented on the Lez river.

• Work on morphology implies owning river banks, or owners’ agreement, and raise several questions :

– Structure stability and consequences on water level during floods

• This solution leads to important works to produce a significant effect at the reach scale

• Other ways of improvement, notably during emergence phase :

Limiting number of hydropeaks and/or the maximum turbine discharge